PROJECT SUMMARY Craniofacial anomalies account for nearly one-third of all birth defects and are a severe cause of morbidity and mortality in infants. In spite of their prevalence, the underlying genetic and molecular mechanisms causing most craniofacial defects remain largely unknown. The long-term goal of the proposed studies is to define the molecular and genetic pathways that control craniofacial development for the purpose of regeneration and repair, tissue engineering, and the diagnosis of and intervention into congenital craniofacial birth defects. Specifically, this application is focused on the identification of the transcriptional pathways and mechanisms involved in craniofacial development. Recent work has identified a novel genetic model for Pierre Robin Sequence (PRS) in mice. PRS occurs in about 1 in every 800 live births and is characterized by small lower jaw, improperly positioned tongue and posterior cleft of the palate, which can result in upper airway obstruction and feeding difficulties. Previously, it was thought that PRS occurred as a result of environmental factors that restrict outgrowth of the mandible, but it is now quite clear that this sequence also has an undefined genetic component(s). Mice that carry one mutant allele of Dlx5/6 locus and one mutant copy of the Mef2c allele die at birth from craniofacial defects resembling PRS. The Dlx5/6 locus encodes two Distal-less related homeobox transcription factors, while the Mef2c locus encodes a MADS box transcription factor. In addition to their genetic interaction, MEF2C and Dlx5 cooperate to induce a robust synergistic transcriptional response. The hypotheses underlying this proposal are that Dlx5/6 and MEF2C form a transcriptional complex downstream of endothelin receptor signaling, that this complex is essential for the activation of a subset of genes that are required for craniofacial development, and that mutations or aberrant expression of Dlx5/6-MEF2C target genes contributes to craniofacial anomalies such as PRS. To address these hypotheses, three specific aims are proposed. Aim 1 will define the physical and functional interaction between Dlx5 and MEF2C and how this results in transcriptional activation. The goal is to define the transcriptional mechanisms that control gene expression during craniofacial development. Aim 2 will analyze the craniofacial defects in Mef2c-Dlx5/6 double heterozygotes in detail. The goal is to identify possible cellular mechanisms underlying the mandible and palate phenotypes in double heterozygotes to understand how these processes control palate closure and jaw growth. Aim 3 will identify upstream regulators of Mef2c transcription in craniofacial mesenchyme and will determine whether Mef2c is a direct target of the endothelin signaling pathway, using a transgenic mouse approach. The goals are to place Mef2c into a transcriptional pathway and to identify the immediate transcriptional effectors of endothelin signaling in craniofacial development.